linux-kernel - net_timedelta() affected by settimeofday() (was: [patch 12/13] net: mac80211: Remove silly timespec dance)

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Date:	Thu, 12 Jun 2014 10:35:14 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"John W. Linville" <linville@...driver.com>,
	linux-wireless@...r.kernel.org,
	linux-netdev <linux-netdev@...r.kernel.org>,
	Stephen Hemminger <shemminger@...ux-foundation.org>
Subject: net_timedelta() affected by settimeofday() (was: [patch 12/13] net:
 mac80211: Remove silly timespec dance)

+netdev, Stephen

On Thu, 2014-06-12 at 10:19 +0200, Thomas Gleixner wrote:
> On Thu, 12 Jun 2014, Johannes Berg wrote:
> 
> > On Wed, 2014-06-11 at 23:59 +0000, Thomas Gleixner wrote:
> > 
> > > +	msrmnt = ktime_to_ms(net_timedelta(skb_arv));
> > 
> > This is probably more of a question about net_timedelta(), but is
> > ktime_get_real() really appropriate for duration measurements? Isn't
> > that non-monotonic?
> 
> Well, it's monotonic, but might be affected by settimeofday().

Right, but isn't that odd? Suddenly your delay measurement here might be
minutes, hours, or years if you settimeofday() between timestamping and
calculating the delta. That seems very strange to me, why would that be
the right behaviour in any way?

Now, it seems that there are only two current users of net_timedelta()
(in DCCP) so perhaps it's not too late to change some of this?

Maybe in general the skb timestamp should be based on a different clock
and only adjusted to real time when used in userspace?

johannes


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